Cutting inserts of the type stated above are used in milling tools for face milling. In this application, the cutting inserts are mounted in insert seats in a number of peripherally spaced-apart pockets in a milling cutter body being rotatable around a central axis, which body during machining of a workpiece is set in a translational feeding motion, usually perpendicular to said axis, at the same time as the same is brought to rotate. In this connection, the main cutting edges of the milling cutter are facing radially outward from the rotation axis of the milling cutter body in order to remove chips from the workpiece in a material layer of a desired depth, while the secondary edges of the cutting insert (which by those skilled in the art usually are denominated “wiper edges”) are located in a common plane and directed inward from the peripherical main cutting edges, in order to, in such a way, exert a surface-wiping or surface-smoothing effect on the generally planar surface, which is generated in the workpiece after the chip removal.
For different purposes, the cutting inserts, in practice being most often flat, can be located at different angles in relation to the milling cutter body. Thus, the individual cutting insert may be inclined or “tipped-in” in a negative as well as a positive angle with the rotation axis, seen not only axially but also radially. Generally, the cutting inserts work easier and more efficient at larger axial angles than at small or negative axial angles. However, the strength and geometry of the cutting inserts, for example practicable clearance angles, impose limits on the maximal axial angles that can be realized.
The problems that the present invention aims at solving are related to cutting inserts for face mills. Thus, it has turned out that easy-cutting cutting inserts, i.e., cutting inserts having a positive geometry of the type initially mentioned, which are mounted with large axial angles (>15°) in the milling cutter body, run the risk of breaking into pieces and having a short service life. Among other things, such cutting inserts are frequently damaged mechanically by the fact that a limited portion of the chip surface in the immediate vicinity of the secondary edge or wiper edge at a corner and the transition thereof into the main cutting edge, is peeled off by the hot chips. If such damage, limited per se, arises, the chip, being viscous by the heat, will shortly thereafter adhere to and pull along with it the area of the chip surface being inside, and in such a way peel off large parts of the surface layer of the cutting insert that determines the geometry of the chip surface. By those skilled in the art, such damages are denominated “topslice fractures”. Damages of this type become particularly frequent when the cutting inserts have large edge rounding offs, and when the material that is machined generates large quantities of heat energy.
A conceivable solution to the above-mentioned problem would be to form the top side of the cutting insert in the shape of a planar, smooth surface. In such a way, the cutting insert in the area of the secondary edge would become stronger and easier be able to resist the planing or shearing effect of the chip. However, such a cutting insert would get a drastically deteriorated performance, among other things because the contact length of the chip against the top side would become considerably larger, with increased cutting forces as a consequence.
U.S. Pat. No. 6,050,752 discloses a cutting insert intended for milling, which insert, adjacent to each one of four surface-wiping secondary edges or wiper edges, has a planar surface that is located at a higher level than the surrounding parts of the chip surface. However, in this case, the cutting insert lacks any positive chip-cutting slope surface adjacent to the individual main edge. Thus, a channelled or waved part of the chip surface extends inward from the main edge in a negative chip angle, which is even larger than the chip angle of the corner surfaces.
SE 502196 C2 discloses a cutting insert for milling, more exactly 90° square shoulder milling, the cutting insert including, in the vicinity of each corner, a ridge being raised relative to the rest of the ship surface, the ridge extending from a wiper edge towards the center of the insert. In this case, however, the ridge as well as the wiper edge are displaced laterally in relation to the curved edge portion, which forms the actual corner of the insert as defined by a bisector between two meeting main cutting edges. This implies that the insert in question runs exactly the same risk to be damaged as the above-mentioned inserts, more specifically topslice fractures being initiated at the fragile curved edge portion in the corner.